valve allows normal flow in one direction andrestricted flow in the other. It is often referredto as a one-way restrictor.Figure 6-26, view A, shows a cone-type orificecheck valve. When sufficient fluid pressure isapplied at the inlet port, it overcomes springtension and moves the cone off of its seat. Thetwo orifices (2) in the illustration represent severalopenings located around the slanted circumferenceof the cone. These orifices allow free flow of fluidthrough the valve while the cone is off of its seat.When fluid pressure is applied through the outletport, the force of the fluid and spring tensionmove the cone to the left and onto its seat. Thisaction blocks the flow of fluid through the valve,except through the orifice (1) in the center of thecone. The size of the orifice (in the center of thecone) determines the rate of flow through thevalve as the fluid flows from right to left.Figure 6-26, view B, shows a ball-type orificecheck valve. Fluid flow through the valve fromleft to right forces the ball off of its seat andallows normal flow. Fluid flow through the valvein the opposite direction forces the ball onto itsseat. Thus, the flow is restricted by the size of theorifice located in the housing of the valve.NOTE: The direction of free flow through theorifice check valve is indicated by an arrowstamped on the housing.SHUTTLE VALVEIn certain fluid power systems, the supply offluid to a subsystem must be from more than onesource to meet system requirements. In somesystems an emergency system is provided as asource of pressure in the event of normal systemfailure. The emergency system will usually actuateonly essential components.The main purpose of the shuttle valve is toisolate the normal system from an alternate oremergency system. It is small and simple; yet, itis a very important component.Figure 6-27 is a cutaway view of a typicalshuttle valve. The housing contains three ports—normal system inlet, alternate or emergencysystem inlet, and outlet. A shuttle valve used tooperate more than one actuating unit may containadditional unit outlet ports. Enclosed in thehousing is a sliding part called the shuttle. Itspurpose is to seal off either one or the other inletports. There is a shuttle seat at each inlet port.6-18Figure 6-27.—Shuttle valve.When a shuttle valve is in the normaloperation position, fluid has a free flow from thenormal system inlet port, through the valve, andout through the outlet port to the actuating unit.The shuttle is seated against the alternate systeminlet port and held there by normal systempressure and by the shuttle valve spring. Theshuttle remains in this position until the alternatesystem is activated. This action directs fluid underpressure from the alternate system to the shuttlevalve and forces the shuttle from the alternatesystem inlet port to the normal system inlet port.Fluid from the alternate system then has a freeflow to the outlet port, but is prevented fromentering the normal system by the shuttle, whichseals off the normal system port.The shuttle may be one of four types: (1)sliding plunger, (2) spring-loaded piston, (3)spring-loaded ball, or (4) spring-loaded poppet.In shuttle valves that are designed with a spring,the shuttle is normally held against the alternatesystem inlet port by the spring.TWO-WAY VALVESThe term two-way indicates that the valvecontains and controls two functional flow controlports-an inlet and an outlet. A two-way, slidingspool directional control valve is shown in figure6-23. As the spool is moved back and forth, iteither allows fluid to flow through the valve orprevents flow. In the open position, the fluidenters the inlet port, flows around the shaft ofthe spool, and through the outlet port. The spoolcannot move back and forth by difference of
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